1. Technical Field of the Invention
The present invention relates to a channel assignment method, more particularly relates to a channel assignment method for multi-FA (Frequency Assignment) CDMA (Code Division Multiple Access) cellular systems, which enhances communication quality and enlarges capacity.
2. Description of the Prior Art
In general, CDMA systems may use a plurality of Frequency Assignments (FAs) at the same time. That is, the system is operated in multi-FA environment in which n FAs from FA1 to FAn (n is natural number) are used. FA used by each mobil station is selected according to a predetermined rule, for example, FA is selected by using HASH function in IS-95 (Interim Standard-95) CDMA cellular system.
Methods for assining channel to mobile station are largely divided into two techniques.
One is a method in which each base station manages FAs independently and a channel in only pre-selected FA is assigned to a mobile station. In this case, if a mobile station communicates through i_th (where, i is natural number) FA, FAi, though there is no useful channel in FAi, and there are some useful channels in another FA, FAj (where j is a natural number, 1≦j≦n, j≠i), a channel in FAj is not assigned to the mobile station communicating through FAi.
The other is a method in which each base station totally manages all useful FAs. In this case, a traffic channel of another FA, FAj, different from a pre-selected FA, FAi, can be assigned to the mobile station when there is no useful traffic channel in the FAi, or the traffic channel of FAi is busier than that of the others.
FIG. 1 is a flowchart illustrating conventional channel assignment method. The method illustrated in FIG. 1 describes the former method.
A base station receives a channel assignment request from a mobile station of FAi and then determines whether the request is a handoff or a new call request at step 12. If the request is handoff call request and there is an available traffic channel in the FAi, the base station assigns the traffic channel at step 13. If the request is a new call request, the base station examines the number of occupied channels in FAi at step 14, and assigns a traffic channel only when the number of idle traffic channels which are not used is larger than that of reserved channels N_ho for handoff in FAi at step 15. The number of the reserved traffic channels for handoff is pre-determined. If the number of the idle traffic channel is less than or equal to that of the reserved channel for handoff, that base station rejects the call request at step 16.
FIG. 2 is a flowchart illustrating another conventional channel assignment method. The method illustrated in FIG. 2 describes the later method.
The base station receives a channel assignment request from a mobile station of FAi at step 21 and then determines whether the request is a handoff call request or a new call request at step 22. If the request is a handoff call request and there is a useful traffic channel in the FAi, the base station assigns the traffic channel at step 23. If the request is a new call request, the base station examines the number of occupied channels in FAi at step 24, and assigns a traffic channel only when the number of occupied traffic channels is less than a threshold N_th at step 25. Threshold N_th which is reference for transition of FA is set in a base station. If the number of the occupied traffic channels is equal to or larger than the threshold, the base station searches FAj (1≦j ≦n) of which the number of occupied traffic channel is less than that of the others at step 26, and assigns a traffic channel only when the number of idle traffic channels is larger than that N_ho of reserved channel for handoff in FAi at step 28. If the number of the idle traffic channel is less than or equal to that oN_ho f the reserved channel for handoff, the base station rejects the request at step 29.
In this case, it is possible to solve the unbalance problem of occupied traffic channel among a plurality of FAs in the base station.
However, in CDMA system, quality and capacity of communication is determined by the amount of interference signal. Accordingly, the number of occupied traffic channel can not be a proper reference for measuring quality of communication or amount of traffic. For example, even in the case that the number of occupied traffic channels of i_th FA, FAi, of a cell is less than that of j_th FA, FAj, if adjacent base stations of the cell experience traffic concentration in FAi, interference level of the FAi could be higher than that of the FAj.
Accordingly, there is a problem that communication quality of a FA is lower than that of the others by assigning a channel in the FA of which the number of occupied channel is less than that of the others but strength of interference signal in the FA is larger.
Therefore, a new channel assignment method is required for overcoming the problem.